1. Field of the Invention
This invention relates to channel input/output (I/O) operations, and particularly to the defining of synchronous replication devices within subchannel sets other than subchannel set zero.
2. Description of Background
Channel I/O broadly refers to any I/O architecture that is employed within a computing architecture environment. Generally, channel I/O is implemented by the use of a plurality of dedicated I/O controllers—or processors—working in conjunction with a CPU in order to ensure the efficient transfer of data within a computing system environment. As such, the I/O controller is supplied with enough onboard logic and memory to autonomously handle a CPU's I/O task processing request.
In operation, the CPU will deliver channel programs to an I/O controller, the channel programs providing the I/O controller with the capability to complete any desired I/O task that is necessitated within the system without the burdening the CPU with the performance of the I/O processing task. Upon the completion of the I/O task, or in the event of all error, the I/O controller will communicate the status of the task to the CPU by way of an interrupt.
In configuration, a channel is an independent hardware component that is implemented to coordinate I/O to a predefined set of devices or controllers. Since a channel may support one or more devices or controllers, the channel programs that are associated with a channel will comprise the commands that the channel is responsive to in addition to the commands for the various devices and controllers to which the channel may be associated. Thus, an operating system has only to assemble the I/O commands for a respective channel program and thereafter execute a single I/O machine instruction to initiate the channel program; wherein thereafter the channel will take responsibility of the I/O task until the completion of the task.
Often implemented in conjunction with channel I/O operations, peer-to-peer remote copy (PPRC) describes a protocol that is implemented in order to mirror a primary storage volume to the same or different control unit that is associated with a secondary storage volume. As such, the utilization of synchronous PPRC causes each write operation to the primary storage to be performed at the secondary storage volume as well. The respective definitions of primary and secondary storage volumes each consume a device address within the addressable device space. The secondary storage volume device address is not addressable by application programs and, therefore, its device address reduces the total device addressability for application programs.